a Poincaré-Perron theorem [1]} assures the existence of
two linearly independent solutions presenting these limits, the minimal
solution being the one with smaller modulus for the limit, \(|t_1|\)  or
\(|t_2|\) . The use of a minimal solution demands
a backward procedure to compute the ratios \(b_{n+1}/b_{n}\) ,
from a large value of \(n\)  downward to \(n=0\) , as in Section 5.
However, before this,
it is necessary to solve the characteristic equation
which affords relations between the parameters of the Heun equation.
Some considerations concerning solutions of characteristic equation for infinite series are given in [2]}, [3]}, [4]}.
